feat(billing): ?plan= 深链用一句文案指向目标卡，而不是第三层描边

D13 把 URL 高亮改成纯滚动定位后留下一个空档：桌面上三张套餐卡本来就都在
视口内，滚动等于什么都没指。付费转化路径上，用户从「开通年度会员」的链接
进来，看不到任何东西指向那张卡。

产品在「加一行文案 / 维持现状 / 把描边加回来」里选了第一个。

用卡片自己的套餐名，不用序数——手机端 [data-plan-alias="monthly"] 带
order:-1，序数会指错。新增断言锁住这两点（必须有 note、不得出现序数写法）。

测试 3354 不变，fail 仍 31 且与基线逐条一致；/ 仍 Static；
干净构建 CSS gzip 39,724→39,733。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0193vBv6w5MV2cifdTUu9H5P

frontend/src/components/billing-panel.tsx

@@ -62,6 +62,11 @@ export function BillingPanel({ account: seedAccount, highlightedPlan, initialTab
     setRedeemMessage,
   } = billing;
   const plans = selectMembershipPlans(paymentPackages);
+  /* Named by the card's own title, not by position: on phones the monthly card
+     takes `order: -1`, so an ordinal would point at the wrong card. */
+  const highlightedName = highlightedPlan
+    ? plans.find((product) => planAlias(product) === highlightedPlan)?.name ?? null
+    : null;
   const creditPacks = paymentPackages.filter((product) => product.productType === "credit_pack");
   const credits = account?.credits ?? seedAccount.credits;
 
@@ -119,6 +124,13 @@ export function BillingPanel({ account: seedAccount, highlightedPlan, initialTab
 
         <TabsContent className="membership-panel" value="membership">
           {packagesSettled && !packagesError && !paymentEnabled && <p className="membership-empty">在线支付暂未开放。</p>}
+          {/* A ?plan= deep link no longer tints or outlines the card it points at
+              (D13: one card, one state). On desktop all three cards are visible
+              anyway, so scrolling alone pointed at nothing — this line is the
+              wayfinding, in copy instead of a third border layer. */}
+          {highlightedName && (
+            <p className="membership-plan-note">你从「{highlightedName}」的链接过来的，就在下面。</p>
+          )}
           {plans.length > 0 && (
             <div className="membership-plan-grid">
               {plans.map((product) => {